It is a recognized fact that the quality of continuously cast metal products, as measured in terms of crystal grain size, the relative proportion of equiaxed vs. columnar crystals and micro crystalline segregation will be significantly affected by the degree of superheat of the molten metal stream which is introduced into any given casting device.
The problem has been addressed to a limited extent in the steel industry, through the use of tundish heaters employed in the conventional continuous casting of steel. The tundishes used in these installations are typically rectangular troughs, approximately 5-7 meters in length, about 0.5-1 meter deep and about 1 meter wide. Such tundishes are capable of processing between about 50-400 tons of steel per hour. The molten metal flowing through these tundishes may be heated either by induction, or by the application of plasma torches. In induction heating the technologies employed include "channel furnace type" arrangements or the placement of induction coils next to the vertical walls of the tundish. In conventional steelmaking applications these tundish heaters are primarily used to compensate for the heat losses experienced in the ladle, from which the steel is being fed and a temperature control within about 10.degree.-30.degree. C. is considered to be the ultimate attainable goal.
A substantially different problem is presented in many applications involving the continuous casting of various non-ferrous metals or alloys using, for example, the Hazelett technique, the wheel casting of bars, the roll casting of sheet, as well as in the novel, near net shape casting of steel, using, for example, the SMS process, wheel casting, the Hazelett technique, single or twin roll casting and the like.
In these systems the tundish used to continuously feed the molten metal tends to be much smaller, for example, approximately 0.5-1 m long, approximately 20-200 mm deep and about 100-500 mm wide, than that utilized in typical steel applications. While the precise temperature control of the liquid entering the caster in these latter systems would be even more important than in case of conventional steel casting technology, tundish heaters have not been used in such shallow tundish applications, due to the inherent difficulties that are necessarily encountered in the induction heating of shallow melts, such as are encountered in these applications. Nonetheless, precise temperature control is most desirable in such applications in order to minimize segregation and to improve the microstructure of the continuously cast products.
In essence, three types of problems can be encountered in such shallow tundish feeder arrangements:
First, it is difficult to couple a shallow melt to electromagnetic heating coils, and thus one experiences "end effects" leading to very poor overall thermal efficiency which is unacceptable in commercial applications.
Second, at higher electrical power input levels, often required to achieve the thermal effect desired, one encounters major disturbances of the free surface of the molten metal in the shallow tundish, which can and often does cause eruptions of the molten metal and thereby creates material loss and an unstable flow of metal to the castor device.
Third, induction stirring resulting from high energy inputs, needed to achieve the desired thermal effect, can also introduce undesirable turbulence resulting in flow disturbances, which could interfere with the steady, smooth supply of the liquid metal to the caster device, which is an essential requirement, particularly in sheet or bar caster applications.
The only prior art of which the applicant is aware involves the use of induction or plasma heaters in the tundish itself. The applicant knows of no prior art teaching the use of a separate vessel, connected to the tundish, which can be heated or cooled, without disturbing the metal flow in the tundish itself which is a novel feature of the present invention.
In conventional steelmaking operations, a ladle furnace is employed wherein the temperature of the molten metal is adjusted before feeding to the tundish. However, such ladle furnaces operate in a batch mode, and thus precise temperature control is impossible to achieve.
None of the aforementioned prior art teach the particular combination of features of the present invention which allows one to overcome the aforementioned specific problems.
It is, therefore, a purpose of the present invention to provide a method and apparatus to achieve very precise temperature control of the liquid metal stream entering the continuous caster apparatus, by the use of a tundish and induction heating means, such that the above mentioned obstacles are obviated.